The 30-year-old central defender has played for a number of clubs in Europe including Standard Liege, Newcastle United, Inter Milan and Twente.
Onyewu is Malaga's fourth signing in the last 24 hours after they also landed Paraguay striker Roque Santa Cruz on loan, Argentine striker Javier Saviola and Chile midfielder Manuel Iturra.
Qatari-owned Malaga have been drawn in Champions League Group C along with AC Milan, Zenit St Petersburg and Anderlecht in their first venture into Europe's elite club competition.
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